The invention relates to an inverter for converting a direct voltage into an alternating voltage, having a number of n circuit units connected in series between two outputs, each circuit unit comprising a series connection, having a direct voltage source and a switch with a switching connection, and a unidirectionally conducting circuit element disposed in parallel with the series connection, the inverter having a control unit which operates the switching connections of the switches to render the switches conductive or non-conductive.
Inverters for supplying alternating voltage loads from direct voltage sources are required, for example, in mains-connected photovoltaic installations, in order to convert the direct voltage generated by the solar cells into an alternating voltage of the same frequency and voltage as those of the mains. Similar inverters are also required to ensure the uninterrupted emergency power supply of alternating voltage mains by means of, for example, rechargeable batteries.
It is known to use for this purpose single or multi-phase inverters or pulse inverters. As a rule such inverters require choke elements, filter sections and transformers. Such elements are expensive and limit the maximum efficiency and economic utilization of such converters. It is also known that sinusoidal alternating voltage is approximated without choke or filter members or transformers by the serial connection of direct voltage sources of identical voltage at the rhythm of the required sinusoidal output voltage (G. J. Naaijer, New Type of Transformerless High Efficiency Inverter, Third E.C. Photovoltaic Solar Energy Conference, Oct. 1980). However, this type of circuit is economical only for a rough subdivision of the sinusoidal voltage wave shape, since a switch stage is required for each graduation. However, such rough graduations produce a relatively high harmonics content, which cannot be permitted, particularly in mains-connected systems, or even in certain loads.
An inverter of the kind specified is also known (British Laid-open Specification No. 2 015 834). In that known circuit the voltage values of the direct voltage sources are also equal to one another in the individual serially connected circuit units, so that the number of possible voltage stages is equal to the number of circuit units present. If the synthetized alternating voltage is to be very finely staged, a considerable amount of circuitry is required, resulting in an elaborate control unit.
It is also basically known (U.S. Pat. No. 3,579,081) to use voltage sources of different voltage, in a differently constructed inverter, which requires four bidirectional switch elements per circuit unit for the additive and subtractive superposition of the voltages of their direct voltage sources.